DOI: 10.18195/issn.0313-122x.78(1).2010.205-246 Records of the Western Australian Museum, Supplement 78: 205–246 (2010). An arid zone awash with diversity: patterns in the distribution of aquatic invertebrates in the Pilbara region of Western Australia Adrian M. Pinder1, Stuart A. Halse1, 2, Russell J. Shiel3 and Jane M. McRae1, 2 1Department of Environment and Conservation, PO Box 51, Wanneroo, Western Australia 6946, Australia 2Present address: Bennelongia Pty Ltd, 5 Bishop St, Jolimont, Western Australia 6014, Australia 3School of Earth and Environmental Sciences, University of Adelaide, Adelaide, South Australia 5005, Australia Corresponding author: email: [email protected] Abstract – The Pilbara region of Western Australia is an arid zone with an abundance of wetlands, ranging from springs and river pools to salt marshes, claypans, rockpools and gnammas. The Pilbara is also rich in mineral resources, which are being heavily exploited, and most of the region is under pastoral lease, with both land uses potentially having an impact upon aquatic systems. In order to provide information for improved conservation planning, including environmental impact assessment and reserve system design, a survey of wetland fauna was undertaken as part of a broader biodiversity survey of the region. This paper describes patterns in the distribution of the region’s aquatic invertebrate diversity and identifi es environmental correlates of those patterns. Invertebrates and environmental variables were surveyed at 100 wetlands in spring and autumn between 2003 and 2006. The Pilbara appears to have a particularly diverse fauna for an arid zone, with just over 1000 species collected, an average of 94 species per sample and a maximum sample richness of 226 species. About a fi fth of the fauna is known to date only from the Pilbara. The remaining species are western endemics, occur elsewhere in northern and/or inland Australia, or have continental or wider distributions. Most species seem to be widespread in the Pilbara region but rare and/or restricted elements are present, especially in some permanently fl owing springs, including those in Millstream and Karijini National Parks, and in ephemeral wetlands such as Fortescue Marsh and freshwater claypans. Faunal composition was not strongly correlated with season, drainage divides, stream order or previously recognised bioregional boundaries. A number of assemblages of species with common patterns of occurrence were identifi ed, each showing unique associations with measured environmental variables. Flow, turbidity, salinity, sediments, macrophytes and hydrological persistence are among the environmental gradients most strongly correlated with occurrence patterns in the fauna. INTRODUCTION is currently hampered by very poor knowledge of how aquatic biodiversity is distributed across The Pilbara region of north-western Australia the region (McKenzie et al. 2002). While a few of is a geologically and geomorphically distinct and the more iconic wetland systems are encompassed ancient part of the continent (Taylor 1994). It has by existing reserves (gorge streams in Karijini a harsh climate, with low but mostly seasonal National Park and the springs and deep permanent rainfall, resulting from intense tropical lows river pools in Millstream National Park), the in summer (many of cyclonic intensity). The conservation estate covers just 7.75% of the Pilbara combination of these elements, plus extensive groundwater aquifers with surface expressions, IBRA region in four parks and is not representative has created a diverse array of wetlands, including of the range of wetlands present in the region. claypans and clay fl ats, rockpools in creeks and This survey of aquatic invertebrates was part of rocky outcrops, springs, river pools (many of a broader project (McKenzie et al. 2009), designed which are permanent) and the large endorheic to provide information on the distribution of the Fortescue Marsh system. Land use in the Pilbara is region’s biodiversity for a range of conservation almost entirely pastoralism and mining and these management needs, including reserve system activities place considerable pressure on the region’s design and environmental impact assessment. wetlands. Effective protection of these wetlands Protection of biodiversity is achieved more easily 206 A.M. Pinder, S.A. Halse, R.J. Shiel, J.M. McRae Figure 1 Maps of the Pilbara region showing wetland locations, major rivers, IBRA sub-regions (shaded, with full names) and drainage basins (bounded by bold lines and abbreviated as follows: ASH, Ashburton; OC, On- slow Coast; FOR, Fortescue; PHC, Port Hedland Coast; DGO, De Grey-Oakover; and GSD, Great Sandy Des- ert. Numbers next to sites in the bottom map are site numbers as per Table 1. Distribution patterns of aquatic invertebrates 207 and effi ciently where there is an understanding of tributaries (Figure 1). This area of about 225,000 km2 what it comprises and how it is distributed across includes all of the Pilbara IBRA bioregion as well the landscape (Haila and Margules 1996; Vinson as the some of the northern Gascoyne IBRA region and Hawkins 1998; Gaston 2000; Margules et al. (parts of the Augustus and Ashburton subregions) 2002). It is assisted further by an understanding (Australian Government 2004). A summary of the of how biodiversity patterning is related to region’s landforms, geology and climate is provided environmental gradients to allow extrapolation by McKenzie et al. (2009). beyond the limited area sampled (Nicholls 1989). The Pilbara region coincides largely with the This type of information is essential to designing Pilbara Craton, one of Australia’s major geological an effi cient conservation reserve system (Brooks blocks (Geological Survey of Western Australia et al. 2004; Pressey 2004), undertaking accurate 1990). The Craton is characterised by hard rock threatened species and communities assessments landscapes that were laid down in Archaean times, (Strayer 2006) and enabling the environmental 2.5 to 3.5 billion years ago, making them some of impacts of land management decisions to be the oldest rocks on the planet (Boulter 1986). The assessed in a regional context (Stork et al. 1996). basement is exposed as granite and greenstone Such work also provides information on ecological terrane in the north but overlain by rugged tolerances, which have direct relevance for sedimentary strata, volcanic fl ows and lateritised conservation by providing explanation of changes caps in the south. in animal abundance and distribution (e.g. Pinder Average annual rainfall is < 350 mm, falling et al. 2005) mostly in summer as a result of tropical lows and In Western Australia (WA), comprehensive cyclones that cross the Pilbara coast about every regional scale aquatic biodiversity assessments have second year on average (http://www.bom.gov.au). been carried out in the Carnarvon Basin, south of Average summer rainfall is 150–300 mm across the Pilbara (Halse et al. 2000; Keighery et al. 2000) most of the Pilbara but attenuates south-westward and in the South-west Agricultural Zone (Halse to < 100 mm. Winter rainfall, which occurs as et al. 2004; Lyons et al. 2004; Pinder et al. 2004). cold fronts pass over the region, has an opposing Biodiversity information from these surveys has trend with an average of 50–75 mm in the west contributed to conservation programs such as the and uplands to < 25 mm in the east and lowlands. selection of ‘biodiversity recovery catchments’ for Average maximum temperatures range from intensive management in the wheatbelt (Walshe 21–30ºC in winter to 36–42ºC in summer (Bureau of et al. 2004, Quinlan et al. 2009) and selection of Meteorology Australia 2008). land for the conservation estate in the Carnarvon In late summer 2002/3 (about six months prior Basin (McKenzie et al. 2000, Brandis 2009). The to sampling commencing in August 2003), several dataset presented here has already been used for low intensity cyclones and tropical lows caused conservation value assessment (e.g. Pinder and fl oods in the Ashburton River and upper (but not Leung 2009; Van Leeuwen 2009) and assessment lower) Fortescue River, and minor flooding in of likely environmental impacts of resource the De Grey-Oakover River. In early March 2004, development proposals in the Pilbara. Cyclone Monty caused severe fl oods in most major This paper examines the distribution of aquatic Pilbara river systems (Figure 2), except the Robe invertebrates across the region in relation to a River. In late March 2004, Cyclone Fay also caused range of landscape and environmental variables. severe fl oods, mostly in eastern rivers such as the The main approach taken is to identify groups De Grey and Coongan. Cyclones Clare and Glenda of species (assemblages) with similar patterns of in January and March 2006 resulted in severe distribution across space and time and presumably, fl oods in the Ashburton, Robe, Fortescue and Port therefore, similar responses to environmental Hedland coast Rivers but only minor fl ooding in gradients. The advantage of identifying groups of the De Grey-Oakover system. No cyclones affected species with similar distribution, before analysing the Pilbara in summer 2004/5. Winter rains in the influence of environmental variables on 2005 resulted in minor fl ow-pulses across most of occurrence, is that gradients and thresholds may the Pilbara (but these are not visible at the scale of be identifi ed for the subsets that are masked or Figure 2). underestimated when working with all species together (McKenzie et al. 2004). Wetland settings Many of the wetland systems in the Pilbara are The Pilbara region strongly infl uenced by the rapid movement of water For the purposes of this study, what we call in the landscape resulting from the combination the ‘Pilbara’ region is bounded to the south by of intense rainfall events (especially from cyclonic the main channel of the Ashburton River and to systems) and the largely impervious geology. the east by the De Grey-Oakover River and its In addition, the extensive alluvial, sedimentary 208 A.M.